1. Field of the Invention
The present invention relates to a composition for forming a dielectric layer, a MIM capacitor and a process for its production.
2. Discussion of Background
In recent years, a composition for forming a dielectric layer has been used to prepare by a coating method a dielectric layer for a MIM (Metal-Insulator-Metal) capacitor wherein a dielectric layer containing a dielectric material having a high dielectric constant such as a barium titanate type or a lead titanate type, is sandwiched between bottom and top electrodes made of conductive films of metal, etc., or a dielectric layer to be used for stabilizing the voltage applied to a luminescent layer in an electroluminescence device. Further, in a high frequency device, a composition for forming a dielectric layer is utilized also as a material for preparing a hybrid substrate, since the electric length can thereby be reduced.
As a conventional composition for forming a dielectric layer, a composition having fluidity comprising fine particles of dielectric crystal, glass frit and an organic vehicle, was used, and such a composition was applied to a substrate by e.g. screen printing or a doctor blade method, then dried to remove the organic vehicle and fired to melt the glass frit thereby to obtain a substrate provided with a dielectric layer. However, there was a problem such that if a large amount of glass frit was mixed in order to obtain a dense dielectric layer, the dielectric constant tended to deteriorate, and on the other hand, if the amount of glass frit was reduced, many pores tended to form in the dielectric layer, again leading to deterioration of the dielectric constant. Further, especially in a case where fine particles of lead type dielectric crystal were to be employed, lead would diffuse into the glass frit during firing at a high temperature, and in a case where the content of glass frit was large, decomposition of the dielectric crystal was likely to take place.
In order to solve such problems, Patent Document 1 discloses a process for obtaining a MIM capacitor having a dielectric layer having a construction such that fine particles of dielectric crystal are bonded by using a composition for forming a dielectric layer containing as a binder a hydrophobic resin such as an epoxy resin, an acrylic resin or a polyimide resin. However, there was a problem such that it was difficult to obtain a thick (particularly one having a layer thickness of at least 1 μm) and dense dielectric layer by using a binder containing such a resin as the main component, and it was difficult to obtain sufficient withstand voltage properties or dielectric constant.
On the other hand, Patent Document 2 discloses a composition for forming a dielectric layer comprising fine particles of dielectric crystal and a hydrolysable silicon compound. If such a composition is employed, it is possible to form a dielectric layer by firing at a low temperature of at most 500° C., and it is also possible to improve the dimensional precision of the dielectric layer. However, it was difficult to form a dense dielectric layer by using only the hydrolysable silicon compound as the binder. Further, it was likely that due to the crosslinking shrinkage of the hydrolysable silicon compound, cracks or pinholes were likely to form in the dielectric layer, whereby the withstand voltage tended to decrease, and depending upon the firing temperature, a problem of warpage of the substrate or the like was likely to result.
Patent Document 1: JP-A-2003-11270
Patent Document 2: JP-A-2003-7135